The invention concerns a method of heating metal strips or other coilable strand metal object to an elevated temperature without oxidation of said metal object, which is oxidisable in air at said elevated temperature wherein the metal object is passed in a heating section through a gas tight heating chamber that is made at least partly of an insulating and electrically non-conductive material while being heated by action of at least two opposite walls of the heating chamber through transverse flux induction heating by means of transverse flux induction heating elements located outside of said chamber which contains a protective non-oxidising gas or gas mixture.
The invention also concerns an apparatus for heating a metal strip or other coilable metal object to an elevated temperature without oxidising said metal object which is oxidisable in air or other oxidising gas at said elevated temperature, wherein the apparatus comprises a gas tight heating chamber through which the metal object is provided to be passed, said chamber being made at least partly of an insulating and electrically non-conductive material and containing a protective, non oxidising gas, said heating chamber having entrance and exit ports for the metal object at the ends thereof, and wherein transverse flux induction heating elements are located opposite to each other outside the heating chamber for transverse flux induction heating of said metal object by action of flux induction heating elements through two opposite walls of the heating chamber as the metal object, is being conveyed through the chamber.
Further, the invention concerns a process line forming an integrated furnace for heat treating a metal strip or other coilable strand metal object, including an entry end and an exit end; a passage-way for the strand object extending along the process line from the entry end to the exit end, said passage-way being enclosed against the ambient environment; the process line further including an apparatus for heating said metal strip or other coilable strand metal object to an elevated temperature without oxidising said metal object which is oxidisable in air or other oxidising gas at said elevated temperature, wherein the apparatus comprises a gas tight heating chamber through which the metal object is provided to be passed, said heating chamber forming part of said enclosed passage way and being made at least partly of an insulating and electrically nonconductive material and containing a protective, non-oxidising gas, wherein transverse flux induction heating elements are located opposite to each other outside the heating chamber for transverse flux induction heating of said metal object by action of the flux induction heating elements through to opposite walls of the heating chamber as the metal object is being conveyed through the heating chamber, and a cooling section including a cooling chamber down-stream of said heating chamber, said cooling chamber also forming part of said enclosed passage-way.
Strips of various types of metals and alloys strain harden when they are cold rolled, cold drawn, or cold stretched. Therefore they need to be heated and annealed in order to be re-crystallised. This particularly concerns stainless steel strips but is valid for metals in general. Conventionally, continuous annealing furnaces are used, which employ fuel or radiant electric heating in chambers through which the strip passes to be heated by conduction and/or radiation. The rate of heating is relatively slow, wherefore the overall furnace lengths need to be correspondingly long.
It is also known in the art to employ induction heating for heating metal strip and other strand metal objects. In principle, there exist two types of induction heating techniques; axial induction heating (AIH) and transverse flux induction heating (TFIH).
AIH is effected by passing an electric current through a wire, which is coiled around, but not touching, the metal to be heated. The electric current induces magnetic currents in the metal, whereby the metal is heated. To be heated in this way, the metal has to be essentially magnetic. Thus metals, such as copper, aluminium, and austenitic stainless steel, can not be easily heated by this technique.
TFIH employs electromagnets of opposite poles positioned on opposite sides of the metal to be heated. The action of passing a magnetic field through the metal heats the metal. The metal, in this case, needs to be electrically conductive but needs not to be magnetic. Thus also copper, aluminium, and stainless steel can be heated by this technique. The use of TFIH for heating metal strip is disclosed e.g. in GB 2 155 740 A, U.S. Pat. No. 4,585,916, EP 0 246 660 B1, EP 0 346 547 B1 and EP 0 667 732 A2.
A facility for producing cold rolled or finally annealed stainless steel strip normally includes at least two annealing sections; a preparatory annealing section and a bright annealing section. In the preparatory annealing section, hot rolled coil is heat-treated to make it easier to form during subsequent cold rolling. Because hot rolled austenitic steel strip, for example, will have a surface layer of scale remnant from the hot rolling process it is sufficient to anneal the strip at an appropriate temperature in a continuous annealing furnace which is open to the atmosphere (air). This process is followed by a descaling operation which removes the hot rolled scale and scale formed by the annealing process. After washing and drying the strip is in a condition suitable for cold rolling.
For surface critical intermediate annealing and final annealing, where a high degree of surface reflectivity is required, it is necessary to protect the surface of the strip from oxidation. This is effected in a continuous strand furnace which contains a protective, non-oxidising gas. Such furnaces can employ direct radiant heat aided by the conduction of the protective gas or by indirect radiant heat from a metal retort, which contains the protective gas and which, is externally heated. A main drawback with these prior art methods is that radiant heating, particularly radiant heating via a gas medium, is a slow process. Annealing furnaces of this type therefore are usually relatively long because of the time needed to raise the temperature of the strip throughout its thickness to the desired annealing temperature and in order to maintain an adequate throughput rate. Thus the capital cost of such furnaces is relatively high. In spite of these drawbacks, this type of annealing furnaces are regularly employed also for new installations, while use of the TFIH technique in practice basically has been restricted to the non-ferrous industry, typically for heating copper and aluminium strip materials to moderate temperatures.
It is the object of the invention to suggest a method and provide an apparatus and a process line which enable the above mentioned disadvantages to be overcome. Accordingly, the invention suggests a method as defined in the above preamble in which the metal object to be heated is a stainless steel object that has been cold rolled to a very high degree of surface reflectivity; the cold rolled stainless steel object is passed through said heating chamber and is heated in said chamber to a processing temperature between 700 and 1200xc2x0 C. the cold rolled stainless steel object is maintained at said temperature between 700 and 1200xc2x0 C. for long enough for the steel to recrystallize completely; and the heat treated metal object then is rapidly cooled directly from the processing temperature, in an airtight cooling section through which the non-oxidising gas is passed, to below a temperature of 600xc2x0 C.
Within the above temperature range, for example, austenitic stainless steels may continuously be annealed at temperatures in the range 1050-1200xc2x0 C., the exact choice of temperature for each grade depending on its specific chemistry. In contrast, cold rolled martensitic stainless steels may be softened in the heating chamber in the range 700-800xc2x0 C., again depending upon their specific chemistry.
The protective gas may in principle be any gas that does not oxidise the metal to be heat treated at the annealing temperature or is otherwise reactive but consists suitably of hydrogen or any other reducing gas or gas mixture, e.g. hydrogen mixed with nitrogen or argon.
The invention has been developed in the first place for bright annealing stainless steel strip, which can vary in width from typically 200 mm to 1500 mm. It is the object of the invention that strips with different widths, but with a ratio between broadest and narrowest conceivable strip not exceeding 2:1, can be heated in the apparatus. Therefore the heating enclosure is made so wide that it can accommodate the broadest strip to be heated in the heating line. Further, in order to achieve good heating efficiency of the transverse flux induction heating elements, the inductors may be positioned close or adjacent to the broad sides of the heating chamber or enclosure wherein cooling channels may be provided between the chamber walls and the inductors. The inductor faces may also be insulated from excessive heat by a covering of heat insulating material, which is non-electrically conductive.
As understood from the above the heating chamber is comparatively narrow in the direction perpendicular to the plane of the strip. The value of this dimension depends on the physical characteristics of the strip to be heated but should be as small as possible to effect maximum heating from the inductors. Because of the relatively high rate of heat input attainable with transverse flux induction heating the length of the heating chamber is short in comparison to furnaces employing conventional heat sources. Because the dimensions of the heating chamber are small in comparison with conventionally heated annealing furnaces the volume of protective gas required is correspondingly small adding to the cost efficiency of the process.
The design of the heating chamber may be either a tube (muffle) or an enclosure having entrance and exit ports for the strip, in which chambers the heat insulation structures (boards) at least span the chamber parallel to the plane of the strip, the dimension of the chamber perpendicular to the plane of the strip being relatively small. Typically it has a generally rectangular cross section, but the shape can also be an elongated oval or have other cross-section.
According to an aspect of the invention, which concerns the apparatus mentioned in the preamble, the protective gas contained in said heating chamber at least substantially consists of hydrogen, wherein the assembly which comprises said outer chamber and said transverse flux induction heating elements which are located opposite to each other outside the chamber, is enclosed in a gas tight enclosure which contains a non-explosive gas capable of acting as a safety shroud in case of accidental rupture of the chamber which contains hydrogen.
According to another aspect of the invention, which concerns the process line, the enclosed passage-way also includes an entrance section between said entry end and said heating chamber and an exit section between said cooling chamber and said exit end, wherein at least one gas conduit is connected to said entrance section and at least one gas conduit is connected to said exit section for replenishing losses of protective gas from the integrated furnace by fresh protective gas from a source of protective gas and for maintaining a positive pressure in the furnace to prevent air from entering the system. Provisions also are suggested to control and/or to minimise any flow of gas through the heating chamber.
Further, according to another aspect of the process line when adapted to heating a metal strip, the process line includes at least one sensor for registering the lateral position of the strip edges relative to the TFIH elements and/or to shading plates located on both sides of the heating furnace between the TFIH elements and the heating chamber; a control unit to which the output signals from said at least one sensor is transmitted and in which the output signals are converted; and motions means controlled by said control unit in dependency of said signals transmitted to the control unit and converted there, which motion means serve to move the TFIH elements and/or the shading plates located between the TFIH elements and the heating chamber to maintain a desired lateral position of the strip edges relative to the TFIH elements and/or to the shading plates.
It is also possible, in the process line of the invention; to combine the TFIH with a conventionally heated unit as a pre heating section where a particularly long soak is required in order to dissolve undesirable phases in the microstructure of the steel and/or in order to achieve desired properties in the material.
Further, in the process line of the invention, it is also possible to combine the TFIH with a conventional electrical heating by means of radiating heating elements in an additional or auxiliary, conventional bright annealing furnace section subsequent to or, possibly, before the TFIH bright annealing section.
TFIH, as employed according to the present invention, primarily is a method of rapid heating. This, however, does not exclude that TFIH, according to a conceivable version of the method of the invention, advantageously can be employed also for applications where it is necessary to control the rate at which a metal needs to be heated. Thus, for thin gauge strip, there might be need for a more gradual heat up rate to operating temperature to avoid distortion. This can be achieved by considered choice of multiple inductors, i.e. a low power inductor to heat the metal relatively slowly part way to the decided final temperature, followed by one or more inductors of higher power for the remainder of the heating cycle. For this application, the term gradual therefore is more appropriate than rapid, as far as the heating rate is concerned, although the heating is more rapid than can be achieved by conventional heating units.
AC power is supplied to the inductor or inductors. For ferrous materials the frequency can be anything between 200 Hz to 3000 Hz or more with power ratings up to 3 MW or more. The selection depends on the strip or other strand dimensions and the required or desired throughput rate.
The invention can be employed also for other heating applications than in connection with annealing operations, but its main advantage is where space is limited and/or where a conventional heating unit is costly.
Further characteristic features and aspects of the invention will be apparent from the following description of some embodiments thereof and from the appending claims.